Question
Explain Van't Hoff factor.
✓
Answer
→ Van't Hoff introduced a factor i, known as the Van't Hoff factor, to account for the extent of dissociation or association. This factor i is defined as:
→ Here abnormal molar mass is the experimentally determined molar mass and calculated colligative properties are obtained by assuming that the non-volatile solute is neither associated nor dissociated.
→ In case of association, value of i is less than unity while for dissociation it is greater than unity.
→ Inclusion of van't Hoff factor modifies the equations for colligative properties as follows:
→Relative lowering of vapour pressure of solvent,
$\frac{p_1^0-p_1}{p_1^0}= i \cdot \frac{n_2}{n_1}$
Elevation of Boiling point, $\Delta T_b=i K_b m$
Depression of Freezing point, $\Delta T_f=i K_f m$
Osmotic pressure of solution, $\pi=i n_2 R T / V$
→ $i$ for several strong electrolytes. For $KCI , NaCl$ and $MgSO _4, i$ values approach 2 as the solution becomes very dilute. As expected, the value of $i$ gets close to 3 for $K _2 SO _4$.
→ Here abnormal molar mass is the experimentally determined molar mass and calculated colligative properties are obtained by assuming that the non-volatile solute is neither associated nor dissociated.
→ In case of association, value of i is less than unity while for dissociation it is greater than unity.
→ Inclusion of van't Hoff factor modifies the equations for colligative properties as follows:
→Relative lowering of vapour pressure of solvent,
$\frac{p_1^0-p_1}{p_1^0}= i \cdot \frac{n_2}{n_1}$
Elevation of Boiling point, $\Delta T_b=i K_b m$
Depression of Freezing point, $\Delta T_f=i K_f m$
Osmotic pressure of solution, $\pi=i n_2 R T / V$
→ $i$ for several strong electrolytes. For $KCI , NaCl$ and $MgSO _4, i$ values approach 2 as the solution becomes very dilute. As expected, the value of $i$ gets close to 3 for $K _2 SO _4$.
| Salt | * Values of i | van't Hoff Factor i for complete dissociation of solute | ||
| 0.1 m | 0.01 m | 0.001 m | ||
| NaCl | 1.87 | 1.94 | 1.97 | 2.00 |
| KCl | 1.85 | 1.94 | 1.98 | 2.00 |
| $MgSO_{4}$ | 1.21 | 1.53 | 1.82 | 2.00 |
| $K_{2}SO_{4}$ | 2.32 | 2.7 | 2.84 | 3.00 |
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